Hotmelt pressure sensitive adhesives (HMPSAs) are compositions that combine the properties of hotmelt adhesives with those of pressure sensitive adhesives. Hotmelt adhesives are solids at room temperature, melt at elevated temperatures to coat on a substrate, and regain their solid form on cooling. Pressure sensitive adhesives are aggressive and permanently tacky at room temperature and adhere to surfaces by application of light finger pressure. The combination of these properties provides compositions that melt at elevated temperatures and cool to form a permanently tacky solid coating that adheres on contact. These compositions are commonly applied to various substrates, such as paper, fabric, metal, and plastic films that are then converted into a large number of different products, especially pressure sensitive adhesive tapes and labels. These pressure sensitive adhesive products have a broad field of application in the automobile industry, e.g., for fastening or sealing, in the pharmaceutical industry, e.g., for bandages or transdermal drug delivery systems, or in the packaging industry, e.g., for sealing, bonding or labeling.
A good workable HMPSA must exhibit high cohesive strength at room temperature, low shrinkage on substrates, retention of pressure sensitive properties during storage and use, and a relatively fluid viscosity at typical coating temperatures (e.g., from about 80° C. to about 180° C.). Although very low molecular weight polymers will yield hotmelt adhesives with sufficient fluidity, the resulting adhesives lack cohesive strength. Very high molecular weight polymers give better cohesive strength, but are too viscous at the common application temperatures to be easily coatable on substrates. They must be extended with a high proportion of low molecular weight oils or resins to reduce the viscosity. The addition of low molecular weight oils or resins in turn detracts from the cohesive strength and heat resistance. In order to increase the cohesion, therefore, a high molecular weight is essential. To avoid these problems, polymers of moderate molecular weight have been made with various functional groups which undergo crosslinking reactions by actinic radiation. In this manner, the cohesion of acrylic PSAs can be raised by means of sufficient crosslinking.
There is an ongoing demand and a continuing need in the art for UV-crosslinkable acrylic polymers that are hot melt processable, as well as methods of manufacturing such polymers. The current invention addresses this need by providing polymers that are functionalized with pendant UV photoinitiators and, following the coating operation, are crosslinked on the substrates as acrylic HMPSAs .